(1) Field of the Invention
The present invention relates to an optical transmission medium made of a drawn synthetic resin, and more specifically, it relates to a synthetic resin optical transmission medium having a continuous refractive index gradient distributed from the peripheral portion to the central portion in the cross section of the optical transmission medium itself and having excellent heat resistance. The invention also relates to a preparation method of the synthetic resin optical transmission medium.
(2) Description of the Prior Art
Heretofore, as optical fibers for transmitting light, there are optical fibers made of a quartz glass and those made of plastics. In the fibers made of the quartz glass, transmission loss is very small. Accordingly, they are widely used for long-distance optical transmission and data transmission. On the other hand, in the optical fibers made of plastics, the transmission loss is larger than that of the optical fibers made of the quartz glass. However, the optical fibers made of the plastics have advantages in that they are light in weight, excellent in flexibility and large in diameter, and can be connected easily with various elements because they are worked easily. In addition, they are inexpensive, so that they are used for short-distance transmission such as data linkage and various sensors.
The optical fibers made of plastics are classified into a GI type (graded index type) having a continuously varied refractive index distribution and an SI type (stepped index type) composed of two different layers called a core and a clad having respectively different refractive indexes. The former one is expected in the use for short-distance large capacity communication with the advantage of its wide transmission band, and the latter one is used for optical transmission and sensors.
Most plastics optical fibers presently used are of the SI type in which the main component is poly(methyl methacrylate) having high transparency, and the glass transition point of this poly(methyl methacrylate) is as low as about 100.degree. C. So that, these plastics optical fibers cannot be continuously used at high temperatures. In recent years, it has been attempted to use the plastics optical fibers under such high-temperature conditions as in microwave ovens or engine rooms of cars. Therefore, in order to improve the heat resistance of the plastics optical fibers, various investigations have been made. For example, in order to improve the heat resistance of poly(methyl methacrylate), there are disclosed a method in which methyl methacrylate and N-arylmaleimide are copolymerized, a method in which methyl methacrylate and styrene or vinyltoluene and maleic anhydride are copolymerized, a method in which polycarbonate is used as a core, a method in which a protective layer is formed outside a clad, and a method in which larger molecules are used in the alcohol moiety of methacrylate (U.S. Pat. No. 4,576,438).
However, these attempts could not improve the heat resistance of the plastics optical fibers to such a level as to withstand the above-mentioned high-temperature conditions.
In addition, most of these attempts are directed to the improvement of the SI type plastics optical fibers having two layers of the core and the clad, and in the GI type having the continuous refractive index distribution, investigation have scarcely been made.
In manufacturing the SI type plastics optical fiber, a melt spinning method in which two kinds of fused polymers are extruded through a nozzle, and this method comprises a polymerization step to prepare polymers having uniform compositions and a subsequent step to spin two or more kinds of polymers by melt extrusion to form two refractive index steps.
On the contrary, in the GI type plastics optical fiber in which a monomer composition continuously changes in the radial direction of the cross section of fibers, it is difficult theoretically to employ the above-mentioned method. So that, such a method is commonly used that, after the formation of a preform having a refractive index distribution, spinning is carried out without changing the condition of distribution. In the case that the copolymerization with a monomer having a high glass transition point is carried out so as to impart the heat resistance to the fiber, it is necessary to employ a quite different polymerization method, or it is impossible to form the refractive index distribution therein. For these reasons, the preparation of the GI type plastics optical fiber has been difficult.
Accordingly, it has been hitherto required to produce a synthetic resin optical transmission medium of refractive index distribution type which can be used at high temperatures and which has a continuous refractive index distribution.